UNITED NATIONS -- Lakhdar Brahimi is on a mission -- one the new joint UN and Arab League envoy to Syria himself admits could be "impossible."

To succeed, he must stem 18 months of fighting between opposition forces and the regime of Syrian President Bashar al-Assad while overcoming sharp differences within the UN Security Council on how to deal with the crisis.

Brahimi is considered a veteran troubleshooter, and the veteran Algerian diplomat is armed with experience. He made his name by mediating the peace plan that ended Lebanon's civil war and has served as the UN's representative for Afghanistan and Iran.

His latest test is beginning in Cairo, where he's meeting with Egyptian and Arab League officials before heading to Damascus.

Upon being handed the reins last month by UN Secretary-General Ban Ki-moon, Brahimi was optimistic that he was up to the task.

"Secretary-General, I think, when you called me, I told you that I was honored, flattered, humbled, and scared. I am still in that frame of mind. I will definitely give this my very, very best," Brahimi said. "I know a few people in Syria and in the region. I have already spoken a little bit about the situation there and about how I was going to serve the United Nations and yourself, the Arab League, and [Arab League Secretary-General] Dr. Nabil Elaraby -- but above all, the Syrian people."

'Nearly Impossible'

In an interview with BBC, Brahimi on September 3 described his mission as "nearly impossible."

Success would have to come where his prominent predecessor failed: former UN Secretary-General Kofi Annan bowed out as special envoy to Syria on August 2 after failing to find consensus for his peace plan.

There are signs that in the weeks since Brahimi's August 17 appointment the situation has degraded even further.

Newly arrived Syrian refugees sit on the road after crossing the border from Syria into Jordan, near the town of Ramtha, in early September.

More than 5,000 people were killed in August alone, according to the pro-rebel organization Syrian Observatory for Human Rights. At least 246,267 Syrians have fled the country, according to the latest figures from the UN High Commissioner for Refugees. And the UN last week called for $347 million in aid for Syria, double the previous goal that was met only halfway by donors.

Brahimi's appointment has been welcomed by Syria as well as by Russia and the United States, whose differences were key to previous failures to forge a peace plan. But Brahimi has also irritated both parties to the conflict in Syria itself; his statement that he had not yet determined if Assad must go infuriated the opposition, and his declaration that Syria is in a state of civil war prompted a rebuke from Assad's regime.

'Star Power'

Jeffrey Laurenti, a senior fellow at New York-based The Century Foundation, says Brahimi is at a disadvantage to his predecessor when it comes to "star power."

While Annan could go directly to U.S. Secretary of State Hillary Clinton and Russian Foreign Minister Sergei Lavrov to negotiate, for example, Laurenti says Brahimi will likely have to go through intermediaries.

On the other hand, Laurenti says Brahimi has the advantage of being a "compatriot" of the Arab world. This is a "card that [Brahimi] is uniquely able to play" and could allow him to elicit more honest information from the rebels and Syrian regime, he adds.

Smoke rises from the El-Edaa district of Aleppo after missiles were launched by a Syrian Air Force fighter jet on September 5.

Laurenti says that the last month of conflict, which has seen aerial bombardments by the Syrian regime and a sharp uptick in violence, has changed the dynamic for the players involved.

"In August, [Assad] arguably had what for him might have been a good month -- for the Syrian people a terrible month -- but he seems to have altered this spreading sense that his departure is inevitable," Laurenti says. "And that this could be a long, drawn-out fight, and could end up in a draw. If the Russians think that he could still fight this to a draw, they aren't going to come aboard with Brahimi any time soon, I would think."

'Toolbox' But No Plan

Brahimi has told media that he has a "toolbox" at his disposal, but he -- along with UN diplomats -- has emphasized that he does not yet have a concrete plan to end the conflict.

The key, according to Laurenti, could come when both Assad's regime and the rebels have exhausted their resources and lost enough popularity to come to the negotiating table.

That's where Brahimi comes in.

"There will be a certain period of time undoubtedly where Brahimi seems to be futilely shuttling back and forth between the sides, and then the great powers [and regional] patrons outside. And then at a certain moment, things begin to shift," Laurenti says.

"These moments come," Laurenti adds, "and Brahimi has been party to that moment in Lebanon 20 years ago."